Dumb Switchgear Question

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MD88

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Hey everybody,

Just a really quick, and dumb question about LV and MV switchgear...

It seems to me that LV switchgear consists of almost moulded-case style breakers and you plug a trip unit into them, which interprets info from CTs integral to the breaker. You don't (and indeed can't?) use relays to control the breakers?

MV switchgear however, needs CTs and relays to control them?

My question stems from a seminar I attended with SEL (Schweitzer) feeder protection relays (specifically the 751A)... We can obviously use these on MV gear, but can we also use it on LV? If so, how does it fit in with the trip unit configuration... Ah I'm so confused.

Thanks!!
 
As Ron said, LV breakers usually have 'shunt trip' coils which can be controlled by a protective relay just like the Trip coil of a MV breaker. In fact, there are also LV breakers available with Close coils (these are usually called electrically or motor operated breakers).

So just about anything you protective relaying you do with MV can also be done with LV breakers.
 
The real issue is, LV breakers (Power Breakers, not often MCCBs) can be easily built with many of the features you get in protective relaying, whereas in MV systems, it would be expensive. Since you are typically looking at much more critical applications in MV gear, you are going to want coordinated relaying anyway. So no point in making the MV breakers more expensive than they already are.

But as the others said, there is no reason why you CAN'T use protective relays on LV gear, it's really just a matter of how much you are willing to spend.
 
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